Rework how 'twin' Refs are represented in GlobalNamespace

A 'twin' ref is created when a name reference in the AST is both a 'get' and 'set', like 'a.b' in the following expression:
  `c = a.b = 0;`.

Currently GlobalNamespace creates two unique Ref objects in this case, one "get" and one "set". This CL adds a new "get and set" ref type and creates one such Ref, not two.

Why?
 * overall decrease in code size
 * I think this model is overall easier to understand
 * helps runtime perf & memory usage by consolidating two fields tracking Refs in GlobalNamespace.Name into one
    before: refs were tracked both in a `Set<Ref>` and `Map<Node, ImmutableList<Ref>>`
    after: refs are only tracked in a single `Map<Node, Ref>`

PiperOrigin-RevId: 499523572

Author: lauraharker